Addition polymerization in a homogeneous reaction mixture

ABSTRACT

A method of producing a polymer comprises the steps of forming a heterogeneous reaction mixture comprising at least one addition polymerizable monomer, a supercritical fluid, and a stabilizer, wherein the stabilizer comprises a chain which is soluble in the supercritical fluid and functional end-group which is not polymerizable by a free-radical mechanism, and polymerizing said at least one monomer in the reaction mixture. In a preferred system for polymerizing acrylic monomers, the fluid comprises supercritical carbon dioxide and the stabilizer comprises an acid-functionalized perfluoropolyether.

This application is the national phase of international applicationPCT/GB99/01830 filed Jun. 9, 1999 which designated the U.S, and thatinternational application was published under PCT Article 21(2) inEnglish.

The present invention relates to addition polymers, in particular tomethods of manufacturing such polymers by dispersion polymerisationtechniques.

Acrylic polymers, such as polymethyl methacrylate are well known andwidely used commercially important examples of addition polymers. Theymay be prepared by various methods, including by bulk polymerisation,emulsion polymerisation or polymerisation in solution. Dispersionpolymerisation has the advantage of keeping the viscosity of thepolymerisation mixture low and enabling the morphology of the polymer tobe controlled and so is particularly suitable for producing highmolecular weight polymer beads.

Polymerisation in supercritical fluids, especially supercritical carbondioxide (sCO₂) has been demonstrated to have the advantages of allowinggood control of particle size whilst also producing polymer with a lowresidual monomer concentration. In addition, the CO₂ does notcontaminate the resulting polymer. EP-A0735051 describes thefree-radical polymerisation of styrene which includes heating a monomer,initiator and a free-radical agent in supercritical CO₂.

WO-A-9504085 describes the use of high molecular weight fluorinatedgraft copolymers and block copolymers for use as surfactants in emulsionpolymerisations using supercritical CO₂ as the continuous phase. Theemulsion polymerisation of acrylamide in a sCO₂ continuous phase with anamide-functionalised fluoropolymer used as an emulsifier is described byBeckman et al in Macromolecules 1994 p 312.

In dispersion polymerisation the monomer is dissolved in the reactionmedium and the resulting polymer, which is not soluble in the reactionmedium, must be kept dispersed to enable the polymerisation reaction tobe carried out efficiently and to control the resulting polymer particlemorphology. A stabiliser may be added to the reaction mixture to keepthe polymer produced in the reaction in dispersion. The requirement of astabiliser compound for this purpose is that it is soluble in thereaction medium, e.g. the supercritical fluid and has an affinity forthe polymer. Existing technology uses high molecular weight block orgraft copolymers which can effectively wrap and coat the growing polymerparticle, thus maintaining a stable dispersion and facilitating controlof the reaction.

Dispersion polymerisation in s-CO₂ using block copolymers is describedin ACS Polymer Preprints 1997, p400; Macromolecules 1995 28 p.8159(DeSimone et al). The use of graft copolymers as dispersionpolymerisation stabilisers is described in Macromolecules 1997 30 p. 745(Beckman et al) in which the stabiliser described has perfluoropolyetherchains grafted onto an acrylic backbone. DeSimone describes the use of amethyl methacrylate-terminated polydimethyl siloxane polymer as astabiliser for the dispersion polymerisation of methyl methacrylate(Macromolecules 1996 29 p.2704). This stabiliser has a polymerisableend-group and thus remains bound into the structure of the resultingpolymer.

The stabiliser materials described in the prior art are usually requiredto be used at high concentration (typically 1-2% or more w/w based onmonomer) and, since they are relatively complex molecules to prepare,they are relatively expensive materials to use. Another problemassociated with the use of these stabilisers is that they tend to remainin the finished polymer and can be difficult to remove completely. Alsothey may not be completely recoverable from the reaction, which furtheradds to the expense of using them.

It is therefore an object of the present invention to provide a methodof producing an acrylic polymer by dispersion polymerisation insupercritical fluids which overcomes some of the above-mentionedproblems.

According to the invention a method of producing a polymer comprises thesteps of forming a homogeneous reaction mixture comprising at least oneaddition polymerisable monomer, a fluid reaction medium, and astabiliser, wherein the stabiliser comprises a chain which is soluble inthe fluid and a functional end-group which is not polymerisable by afree-radical mechanism and polymerising said at least one monomer in thereaction mixture.

According to a second aspect of the invention, a stabiliser for use indispersion polymerisation of acrylic monomers in a fluid reaction mediumcomprises a material having a chain which is soluble in the fluidreaction medium and a functional end-group which is not polymerisable bya free-radical mechanism.

The monomer may be any suitable unsaturated compound which is useful inthe formation of addition polymers. Suitable monomers include, but arenot limited to, optionally functionalised or substituted vinyl monomerssuch as styrene, acrylic monomers, vinyl chloride, vinyl acetatesubstituted olefins and maleic anhydride. A preferred group of monomerscomprises esters of acrylic or methacrylic acids, their alkyl esters,and substituted analogues thereof. More than one monomer may be presentif a copolymer product is required. Preferred monomers include alkylacrylates, methacrylates and ethacrylates, especially methyl(meth)acrylate, ethyl (meth)acrylate and butyl (meth)acrylate.

The reaction is preferably carried out in the presence of a free-radicalinitiator. Suitable initiators include azo-compounds such asazobis(isobutyronitrile) (AIBN),azobis(4-methoxy-2,4-dimethylvaleronitrile (commercially available as“V-70”), and peroxides such as dicumyl peroxide and t-butyl peroxide.

The fluid reaction medium may comprise any known fluid in which themonomer is soluble and preferably comprises a fluid which may be broughtinto supercritical state as commonly known in the art. As is known inthe art such fluids may be subjected to conditions of temperature andpressure up to a critical point at which the equilibrium line betweenliquid and vapour regions disappears. Supercritical fluids arecharacterised by properties which are both gas-like and liquid-like. Inparticular the fluid density and solubility properties resemble those ofliquids, whilst the viscosity, surface tension and fluid diffusion ratein any medium resemble those of a gas, giving gas-like penetration ofthe medium.

Preferred fluids include carbon dioxide, di-nitrogen oxide, carbondisulphide, aliphatic C2-10 hydrocarbons such as ethane, propane,butane, pentane, hexane, ethylene, and halogenated derivatives thereofsuch as for example carbon hydrogen trifluoride or chloride and HCF134a,C6-10 aromatics such as benzene, toluene and xylene, C1-3 alcohols suchas methanol and ethanol, sulphur halides such as sulphur hexafluoride,ammonia, xenon, krypton and the like. Typically these fluids may bebrought into supercritical conditions at temperature of between 0-150°C. and pressures of 7-1000 bar, preferably 12-800 bar. It will beappreciated that the choice of fluid may be made according to itsproperties, for example diffusion and solvent properties. The choice offluid may also be made with regard to critical conditions whichfacilitate the commercial preparation of the polymer.

The preferred fluid comprises supercritical carbon dioxide, optionallyin admixture with a further fluid. The advantages of using carbondioxide include the fact that it forms a supercritical fluid atrelatively low temperatures (32° C. at 74 bar), is readily available andeasy to handle and can be removed from the reaction mixture by venting,leaving little residue.

The temperatures and pressures used depend upon the nature of the fluidused and the conditions under which it exhibits supercriticalproperties. The reaction need not be carried out under supercriticalconditions and the fluid may be a liquid when the temperature is belowthe supercritical range of the fluid. Also the temperature and pressureat which the reaction is carried out is dependent upon the nature of theinitiator used, as is known in the art. In a preferred system, for thepolymerisation of acrylic materials in supercritical carbon dioxide, thereaction is preferably carried out at pressures in the range1,000-10,000 psi, more preferably 1,500-7,000 psi at temperatures ofbetween about 0-150° C., preferably between about 40-80° C., e.g. about70° C. when the initiator used is AIBN.

The stabiliser comprises a chain which is soluble in the fluid and afunctional end-group. The fluid-soluble chain may comprisefluoropolymers, siloxanes, polyphosphazenes, polyethylene oxides orother polymer chains which are soluble in the chosen supercriticalfluid. Preferably the stabiliser comprises a functionalisedfluoropolymer, especially a functionalised perfluoro-polyether, which ispreferably terminally functional. Suitable end-groups comprise acarboxylic acid, amide ester, amine, acid chloride, alcohol, phosphateor like group. Preferably the stabiliser comprises a carboxylic acidend-group. A particularly preferred stabiliser comprises a carboxylicacid terminated perfluoro polyether. The stabiliser may bemonofunctional or polyfunctional, e.g. difunctional, howevermonofunctional stabilisers are preferred. The molecular weight of thestabiliser may vary widely, e.g. between about 300 and 10⁶ Daltons (D).We have found that materials having a molecular weight (M_(w)) in therange 1000-10,000 are particularly effective as stabilisers in somepreferred systems. We have found that suitable such stabilisers includecarboxylic acid terminated perfluoro polyether materials such as thosesold under the trade names KRYTOX™ 157 FSL, 157 FSM, 157 FSH by DuPont,GALDEN™ MF300 or FOMBLIN™ DA601 as sold by Ausimont. We have found thatthe use of such materials as a stabiliser allows good control of themorphology of the polymer particles produced and effectively stabilisesthe polymerisation. A further advantage offered by the use of thesematerials is that the stabiliser appears not to become incorporated intothe polymer and may be removed from the polymer relatively easily byventing along with the fluid solvent.

The concentration of stabiliser in the reaction mixture is preferably inthe range 1×10⁻⁵−40 wt % with respect to the monomer concentration, morepreferably 0.01-10%. We have found that the concentration of thestabiliser affects the morphology of the polymer particles produced inthe reaction. By varying the stabiliser concentration, the resultingpolymer may have a morphology which vanes from isolated sphericalparticles of mean diameter 0.5-5 μm to elongated chains of agglomeratedparticles which form open porous structures of high surface area. At lowstabiliser concentrations, nodular morphologies may be formed. At astabiliser concentration of 0.1-35% when the fluid used is supercriticalCO₂, the polymerisation of methyl methacrylate produces well dispersedparticles and the stabiliser may be removed easily by venting. Themorphology produced may also be controlled by controlling the density ofthe supercritical fluid.

The molecular weight of the polymer produced may vary widely e.g. from20,000-400,000 Daltons (M_(w)). We have found that a particularadvantage of the method of the invention using the stabilisers describedabove is that the yield of polymer produced may be relatively high. Forexample, typical yields achievable are at least 85% when the polymermolecular weight is in the range 130,000-300,000.

The polymerisation mixture may include other additives, such as chaintransfer agents for example. Chain transfer agents are commonly used toproduce polymer which is more thermally stable than normalradical-terminated polymer. Suitable chain transfer materials are wellknown and include a range of mercaptans. A further advantage of thepolymerisation method of the invention is that residual chain transferagent may be easily removed from the polymer by venting with the fluidmedium.

The method of the invention is further described in the followingExamples.

EXAMPLE 1

A high-pressure autoclave (60 ml volume) was charged with about 10 g ofmethyl methacrylate (MMA), 1 wt % (by weight of MMA) ofazobis(isobutyronitrile), and 1 wt % of KRYTOX™ FSL, which is acarboxylic acid-terminated perfluoro polyether sold by DuPont. Theautoclave was then pressurised with CO₂ to 200 bar and the temperaturerapidly increased to 70° C. to initiate the polymerisation reaction.After 4 hours, the CO₂ was vented and a fine white polymer powder hadbeen formed at a yield of >95% (based on monomer). The molecular weightof the resulting polymer was determined by gel permeation chromatography(GPC) and the morphology and particle size (when particles were formed)were determined by scanning electron microscopy (SEM). The polymerparticles had a relatively uniform particle size of 2.5 μm and apolydispersity index (PDI) of <2.8.

EXAMPLE 2

The procedure described in Example 1 was followed using a number ofdifferent acid-functionalised perfluoro polyethers as stabilisers at aconcentration of 1%. The stabilisers differed in molecular weight(measured in Daltons). The results are shown in Table 1.

TABLE 1 Stabiliser mol % M_(n) particle diameter Stabiliser wt. (D)yield (kD) PDI average (μm) Galden MF300  850 96.10 47.40 3.10 — Krytox157 2500 96.10 54.40 2.68 2.70 FSL Krytox 157 5000 99.90 54.20 2.85 1.90FSM Krytox 157 7000 97.20 51.50 2.61 1.90 FSH

The results show that a high yield of polymer of high molecular weightand good polydispersivity was obtained using all of the stabilisers.However the low molecular weight stabiliser did not produce aparticulate polymer.

EXAMPLE 3

The polymerisation procedure described in Example 1 was repeated usingFluorolink E™ which is a perfluoropolyether having an Mn of about 2000 Dand terminated at each end with an alcohol group. The resulting polymerhad an Mn of approx. 40 kD, and a PDI of about 3. The morphology of thepolymer was not particulate.

EXAMPLE 4

The polymerisation procedure described in Example 1 was repeated usingFluorolink C™ which is a perfluoropolyether having an Mn of about 2000 Dand terminated at each end with an acid group. The resulting polymer hadan Mn of approx. 35 kD, and a PDI of about 3.5. The morphology of thepolymer was not particulate.

EXAMPLE 5

The polymerisation procedure described in Example 1 was repeated usingFomblin™ DA601 which is a perfluoropolyether having an Mn of about 5000D and terminated at one end with a phosphate group. The resultingpolymer had an Mn of approx. 50 kD, and a PDI of about 2.7. The polymerwas particulate, with average particle size of about 2.7 μm.

EXAMPLE 6 (COMPARATIVE)

The polymerisation procedure described in Example 1 was repeated usingGalden™ HT55 which is an unfunctionalised perfluoropolyether having anMn of about 2000 D. This compound was immiscible with the MMA monomer.The resulting polymer was produced in a yield of about 24% w/w, had anMn of approx. 17.7 kD, and a PDI of about 3.3. The polymer was of suchlow molecular weight it was extracted as a solution in the monomer andso morphology was not studied.

EXAMPLE 7

The procedure of Example 1 was repeated using Krytox™ 157 FSL at varyingconcentrations. The results are shown in Table 2 below.

TABLE 2 Stabiliser conc average particle (% w/w) % yield M_(n) (kD) PDIdiameter (μm) 0 (Comparative) 28.6 10.90 3.6 agglomerated 0.0001 89.232.3 4.0 ″ 0.0002 90.1 42.7 3.40 ″ 0.0008 94.5 50.1 2.80 chains ofparticles 0.01 96.2 54.9 2.64 2.1 0.1 93.2 40.0 2.79 2.9 1.0 95.9 47.72.62 2.9 2.77 92.5 52.2 2.95 2.5 4.18 91.8 65.9 2.43 2.5 10.4 99.9 68.12.45 2.6 16.0 91.9 62.7 2.62 2.6 32.0 85.3 89.9 1.93 2.0

The results show that the use of Krytox 157 FSL as a stabiliser greatlyincreases the yield of the polymer even at very low concentrations andthat the resulting polymer is in the form of discrete particles atconcentrations as low as 0.01%. At very low concentrations, or in theabsence of stabiliser, a foam-like morphology is seen on a microscopicscale. Increasing the concentration of the stabiliser tends to increasethe molecular weight of the polymer.

EXAMPLE 8

The polymerisation procedure in Example 1 was repeated using Krytox 157FSL as stabiliser and butyl mercaptan as a chain transfer agent theresults are shown in Table 3.

TABLE 3 % w/w butyl mercaptan % yield Mw (kD) M_(n) (kD) PDI 0.01 96.8119.5 42.1 2.84 0.04 93.6 119.2 48.8 2.40 0.06 86.9 107.4 36.3 2.96 0.0880.7 75.5 27.2 2.78 0.20 75.5 47 21.3 2.20 0.26 66.7 36.6 16.7 2.19

The results show that polymerisation in the presence of chain transferagents is possible using the method of the invention and that themolecular weight of the polymer produced falls with increasingconcentration of the mercaptan, as would be expected. Also, theexperiment demonstrates that the efficiency of the chain transfer agentappears to be increased compared to dispersion polymerisation inconventional fluid media with alternative stabilisers. The chaintransfer constant of a methyl methacrylate/butyl mercaptan system isgiven in The Polymer Handbook (Brandrup & Immergut, John Wiley pub) 3rdEdn, page ii/135 as 0.66. The chain transfer constant was estimated fromthe experimental results in Table 3 from the slope of a plot of [butylmercaptan]/[MMA] versus 1/X_(n), where X_(n)=M_(n)/(molecular weight ofMMA) as 8.0. Therefore the method of the present invention may allowless chain transfer agent to be used than in conventional methods. Sincemercaptans may have an unpleasant odour, it is beneficial to use areduced quantity.

EXAMPLE 9

A series of polymerisations were conducted using the general procedureoutlined in Example 1, but at varying pressures. The results are shownin Table 4, below. The results show that the molecular weight andparticle size may be controlled by controlling the pressure of thereaction and therefore the density of the supercritical CO₂.

TABLE 4 Pressure density CO₂ % Mw M_(n) average particle (psi) (g/cm³)yield (kD) (kD) PDI diameter (μm) 1500 0.257 99.30 257.1 74.0 3.47 3.52100 0.455 99.9 189.2 66.8 2.83 3.5 2800 0.626 96.0 150.6 47.0 3.15 2.53514 0.713 92.6 132.8 40.3 3.30 2.8 4000 0.753 99.9 139.9 47.2 2.96 2.84436 0.786 91.6 107.4 43.9 2.78 2.4 4900 0.807 80.5 102 46.8 2.20foam-like

What is claimed is:
 1. A method of producing a polymer comprising thesteps of forming a homogeneous reaction mixture comprising at least oneaddition polymerisable monomer, a fluid reaction medium, and astabiliser, wherein the stabiliser comprises a perfluoro polyether chainwhich is soluble in the fluid reaction medium and a carboxylic acidfunctional end-group which is not polymerisable by a free-radicalmechanism, and polymerising said at least one monomer in the reactionmixture.
 2. A method as claimed in claim 1, wherein said stabiliser ispresent at a concentration in the range 1×10⁻⁵−40 wt %.
 3. A method asclaimed in claim 1, wherein said fluid reaction medium comprisessupercritical carbon dioxide.
 4. A method as claimed in claim 1, whereinsaid at least one monomer is selected from the group comprising estersof acrylic or methacrylic acids, their alkyl esters, and substitutedanalogues thereof.
 5. A method as claimed in claim 1, wherein thereaction mixture further comprises a chain transfer agent.